Thermal stabilization of polyesters



United States Patent US. Cl. 260-453 5 Claims ABSTRACT OF THE DISCLOSURE A thermal stabilized polyester composition comprising a saturated linear polyester resin containing an additive selected from the group consisting of silicon isocyanate, phosphorous isocyanate, and phosphoryl isocyanate.

This invention relates to highly polymeric linear polyester resins that possess improved thermal stability and to a method of producing same.

Saturated linear polyester resins can be prepared by first carrying out a condensation reaction between a suitable dicarboxylic acid or ester thereof with a diol to form a prepolymer. The resulting prepolymer is then polycondensed to form the desired polyester resin. When an ester of a dicarboxylic acid is used as the starting material, it is first reacted with a diol in the presence of a transesterification catalyst by means of an ester-interchange reaction; whereas when a dicarboxylic acid is used as the starting material, it is first subjected to a direct esterification reaction with the diol in the presence of what is generally called a first stage additive or ether inhibitor. In either instance, the resulting reaction product, which may be generally described as a polyester prepolymer, is then polycondensed in the presence of a polycondensation catalyst to form the polyester resin.

For example, in the case of the transesterification method of preparing polyethylene terephthalate, ethylene glycol is reacted with dimethyl terephthalate to form the polyester prepolymer which is comprised mainly of bis-2-hydroxyethyl terephthalate; or in the direct esterification method, ethylene glycol is reacted with terephthalic acid to form the resulting polyester prepolymer which is then polycondensed to form the desired polyester resin.

Saturated linear polyester resins, such as polyethylene terephthalate and many others. are widely used in the production of films and fibers and the like. However, it is generally known that such polyester products degrade when exposed to heat for a substantial period of time. Such degradation is particularly a problem in the extrusion and spinning processes of the finished resins. Additionally, the fibers produced from such resins are extensively used in the textile field and, as a result of this application, are subjected to rather extreme temperatures in the process of washing, drying, and ironing. Therefore, it is highly desirable that the polyester resin composition possess as much stability at high temperature as possible.

Therefore, it is an object of the present invention to prepare a highly polymeric saturated linear polyester resin composition which exhibits improved thermal stability.

Another object of the present invention is to provide a method of preparing saturated linear polyester resin exhibiting such a high degree of thermal stability.

These and other objects are accomplished in accordance with the present invention with a stabilized polyester composition comprising a saturated linear polyester resin containing an additive selected from the group consisting of silicon isocyanate, phosphorous isocyanate, and phosphoryl isocyanate.

The preparation of polyester via the ester-interchange ice reaction is generally carried out with a molar ratio of diol, such as ethylene glycol, to an ester of a dicarboxylic acid, such as dimethyl terephthalate, of from about 1:1 to about 15:1, respectively, but preferably from about 1.2:1 to about 2.6:1. The transesterification or ester-interchange reaction is generally carried out at atmospheric pressure in an inert atmosphere such as nitrogen initially at a temperature ranging from about l25250 C., but preferably between about C. and 200 C. in the presence of a suitable transesterification catalyst. For example, the transesterification catalyst used may be lithium hydride or zinc acetate at a concentration ranging from about 0.01% to about 0.20%, based on the weight of the ester of the dicarboxylic acid used in the initial reaction mixture. During the first stage of this reaction, methyl alcohol is evolved and is continuously removed by distillation. After a reaction period of about 1-2 hours, the temperature of the reaction mixture is raised to from about 200 C. to about 300 C. for approximately 1-3 hours in order to complete the reaction, so as to form the desired polyester prepolymer and to distill any excess glycol.

In the case of the direct esterification method of preparing saturated linear polyester resins, the reaction is generally carried out with a molar ratio of diol, for example ethylene glycol, with a dicarboxylic acid, such as terephthalic acid, of from about 1:1 to about 15:1, but preferably from about 1.2:1 to about 2.6:1. The direct esterification step is generally carried out at temperatures ranging from about ISO-280 C. in the absence of an oxygen containing atmosphere at atmospheric or elevated pressure for about 2-4 hours to form the desired polyester prepolymer. For example, the reaction may be carried out in an atmosphere of nitrogen.

The first stage of the direct esterification method is generally carried out in the presence of a suitable additive or buffer, such as triethylamine, at a concentration ranging from about 5 X 10' mole to about 5X 10* mole of additive per mole of dicarboxylic acid in the initial reaction mixture.

conventionally, the polycondensation stepin the preparation of the subject polyester resins by either the transesterification method or direct esterification method, is generally accomplished by adding a suitable polycondensation catalyst to the polyester prepolymer and heating the blend thereof under reduced pressure of from about 0.5 to 20 mm. of mercury while being agitated at a temperature of from about 260-325 C. for from 2-4 hours. For example, the polycondensation catalysts suitable for use are antimony trioxide and antimony sec-butoxide, at concentrations ranging from about 0.01% to about 0.2%, based on the weight of the polyester prepolymer to be polycondensed.

In the practice of the present invention, the abovedefined isocyanate thermal stabilizers can be incorporated in the resin composition at various stages. For example, in the preparation of polyester resin. the present thermal stabilizers can be suitably mixed in the polyester resin reactants before commencing the reaction, or at any stage after the initial reaction has begun, if indicated. However, in most instances, it has been found that it is preferred to mix or blend the present thermal stabilizers in the polyester resin immediately after the polycondensation step has been completed.

It has been found that silicon isocyanate, phosphorous isocyanate, and phosphoryl isocyana-te are eifective as thermal stabilizers in polyester resin compositions when employed in amounts ranging from about 0.01% to about 0.5%, based on the weight of the polyester resin. Usually, it has been found that concentrations ranging from about 0.02% to about 0.3% is preferred in most instances. However, when indicated, concentrations less or greater than the above can be used, but the eflectiveness thereof is generally reduced.

The following examples of several preferred embodiments will further serve to illustrate the present invention, although it will be understood that these examples are included merely for the purpose of illustration and are not intended to limit the scope of the present invention. All parts are by weight, unless otherwise indicated.

EXAMPLE I A blended mixture comprising 474 g. of terephthalic acid, 288 mls. of ethylene glycol and 149 mls. of triethylamine was charged into a reaction vessel equipped with a nitrogen inlet, a Dean-Starke separating apparatus, heating means, and stirring means. The reaction mixture was agitated and the temperature was raised to about 197 C. under a nitrogen blanket at atmospheric pressure. At about 190 C., a water-triethylamine azeotropic mixture started to distill off. The azeotropic mixture was continuously separated by means of the Dean-Starke apparatus, and the triethylamine recovered was continuously returned to the reaction vessel. The reaction mixture became almost clear. Then, the temperature was allowed to rise to about 220 C. over a one hour period to form a polyester prepolymer. The prepolymer product was allowed to cool under an atmosphere of nitrogen.

EXAMPLE II EXAMPLE III Fifty grams of the prepolymer product of Example I was mixed with 0.02 g. of antimony sec-butoxide and placed in a reaction vessel. The reaction mixture was heated at about 280 C. under reduced pressure of from about 0.05 to about 0.1 mm. of mercury while under agitation for about 2 hours to bring about the polycondensation of the prepolymer and formation of a polyester resin. At the end of the polycondensation reaction. 0.02 g. of silicon isocyanate, Si(NCO) was stirred into the polyester resin product and then cooled. The resulting polyester resin composition had an original intrinsic viscosity of 0.73, a degraded intrinsic viscosity of 0.65, and the percentage broken bonds was calculated as 0.077.

EXAMPLE IV Fifty grams of the prepolymer product of Example I was mixed with 0.02 g. of antimony sec-butoxide and placed in a reaction vessel. The reaction mixture was heated at about 280 C. under reduced pressure of from about 0.05 to about 0.1 mm. of mercury while under agitation for about 2 hours to bring about the polycondensation of the prepolymer and formation of a polyester resin. At the end of the polycondensation reaction, 0.02 g. of phosphorous isocyanate, P(NCO) was stirred into the polyester resin product and then cooled. The resulting polyester resin composition had an original intrinsic viscosity of 0.63, a degraded intrinsic viscosity of 0.59, and the percentage broken bonds was calculated as 0.042.

EXAMPLE V Fifty grams of the prepolymer product of Example I was mixed with 0.02 g. of antimony sec-butoxide and placed in a reaction vessel. The reaction mixture was heated at about 280 C. under reduced pressure of from about 0.05 to about 0.1 mm. of mercury while under agitation for about 2 hours to bring about the polycondensation of the prepolymer and formation of a polyester resin. At the end of the polycondensation reaction, 0.02 g. of phosphoryl isocyanate, OP(NCO) was stirred into the polyester resin product and then cooled. The resulting polyester resin composition had an original intrinsic viscosity of 0.80, a degraded intrinsic viscosity of 0.68, and the percentage broken bonds was calculated as 0.091.

EXAMPLE VI A mixture comprising 600 g. of dimethyl terephthalate, 396 mls. of ethylene glycol, and 0.24 g. of lithium hydride was charged into a reaction vessel equipped with a nitrogen inlet, heating means and stirring means. The reaction mixture was agitated and heated at atmospheric pressure at 198 C. under a nitrogen blanket. The reaction mixture was held at about 198 C. for about two hours, during which time by-product methyl alcohol was distilled off. Then, the temperature of the reaction mixture was allowed to rise to 230 C. over a period of about one hour to distill off any remaining by-product methyl alcohol and ethylene glycol and form a polyester prepolymer. The prepolymer product was allowed to cool under an atmosphere of nitrogen.

EXAMPLE VII Fifty grams of the prepolymer product of Example VI was mixed with 0.02 g. of antimony trioxide and placed in a reaction vessel. The reaction mixture was heated at about 280 C. under reduced pressure of from about 0.05 to about 0.1 mm. of mercury while under agitation for about 3 hours to bring about the polycondensation of the polyester prepolymer and formation of a polyester resin. The polyester resin formed had an original intrinsic viscosity of 1.13, a degraded intrinsic viscosity of 0.76, and the percentage broken bonds was calculated as 0.169.

EXAMPLE VIII Fifty grams of the prepolymer product of Example VI was mixed with 0.02 g. of antimony trioxide and placed in a reaction vessel. The reaction mixture was heated at about 280 C. under reduced pressure of from about 0.05 to about 0.1 mm. of mercury while under agitation for about 3 hours to bring about the polycondensation of the polyester prepolymer and formation of a polyester resin. At the end of the polycondensation reaction, 0.02 g. of silicon isocyanate, Si(NCO) was stirred into the polyester resin product and then cooled. The polyester resin formed had an original intrinsic viscosity of 0.67, a degraded intrinsic viscosity of 0.62, and the percentage of broken bonds was calculated as 0.052.

In the above examples, the original intrinsic viscosity values of the polyester resin products were obtained by measuring the intrinsic viscosities of the resin compositions as produced.

The degraded intrinsic viscosity values were determined by the following procedure: The polyester resin composition was ground and passed through a 10 U.S.S. mesh screen and dried at C. in vacuo for 16 hours, then cooled in a desiccator. Two to three grams of this dried resin was then placed in a test tube which was then inserted into an aluminum block preheated to 280 C. (:05 C.). The block was then sealed and evacuated to 0.1 mm. of mercury. After holding for about 10-15 seconds, the block was filled with dried, oxygen-free nitrogen gas. This vacuum-nitrogen purge was then repeated for a total of three times; the entire process took 5-7 minutes. Then, the resin sample was left in the heated block for an additional two hours under a slow stream of nitrogen. After this two-hour period, the resin sample was removed from the block and placed in a desiccator which was first evacuated and then filled with nitrogen. The intrinsic viscosity of the resin product was then determined and such an intrinsic viscosity value is noted in the examples above as the degraded intrinsic viscosity.

The percentage broken bond valuesindicated in the above examples were calculated by the use of the following equation:

Percent broken bonds=l:( y X 9.6)(10 The values for K and a may be found in the literature, such as Conix, A., MakromoL, Chemie 26, p. 226 (1958), wherein K=0.00021 and :0.82.

All of the intrinsic viscosity determinations of the polyester resin products produced in the above examples were determined in a 60% phenol and 40% tetrachloroethane solution, wt./wt., at 30 C., according to conventional laboratory procedure.

The results in the above examples indicate that the present additives, when added to saturated linear polyester resins, act to stabilize or reduce the degradation effects of higher temperatures upon such polyester resins. The change in intrinsic viscosity or the difference between the original intrinsic viscosity and the degraded intrinsic viscosity is a direct measure of the heat stabilizing etfect that the present thermal stabilizers have upon polyester resins and can be readily calculated from the above results.

When the controls above, Examples II and VII, are compared with their corresponding examples wherein the same catalyst systems Were used, with the addition of a thermal stabilizer of the present invention, it can readily be seen from the intrinsic viscosity values and the percentage broken bonds values that the present stabilizers act in all cases to limit the amount of degradation that takes place when polyester resin products are exposed to elevated temperatures for prolonged periods of time.

The present invention has been illustrated with particular respect to the stabilization of polyethylene terephthalate. However, the present thermal stabilizers are also effective in stabilizing any saturated linear polyesters and copolyesters; for example, those derived from dicarboxylic acids, such as isophthalic acid, and 4,4'-diphenyldicar- 6 boxylic acid, or ester derivatives thereof, and suitable diols, such as glycols of the series HO(CH OH, where n is 2 to 10.

It will be apparent that various diiferent embodiments can be made practicing this invention without departing from the spirit and scope thereof, and therefore, it is not intended to be limited except as indicated in the appended claims.

We claim:

1. A stabilized polyester resin composition comprising the product formed by incorporating from about 0.01% to about 0.5% of an additive selected from the group consisting of silicon tetraisocyanate, phosphorus triisocyanate, and phosphoryl triisocyanate into a molten saturated linear polyester resin.

2. The composition of claim 1 wherein the polyester resin is polyethylene terephthalate.

3. The composition of claim 1 wherein the additive is silicon tetraisocyanate.

4. The composition of claim 1 wherein the additive is phosphorous triisocyanate.

5. The composition of claim 1 wherein the additive is phosphoryl triisocyanate.

References Cited UNITED STATES PATENTS 2,835,652 5/1958 Haven 260-551 3,054,756 9/ 1962 Holtschmidt et al 260-75 3,098,840 7/1963 Holtschmidt et al. 260-459 3,178,391 4/1965 Holtschmidt et al 260-75 3,296,190 1/1967 Rieschl et al 260- .9 3,334,056 8/ 1967 Fischer et al. 260- 3,403,128 9/1968 Berndt et a1 260-459 3,113,196 12/1963 Fielding 260-75 HOSEA E. (TAYLOR, JR., Primary Examiner US. Cl. X.R. 260-75 

